异步加载js

    //异步加载js  
    function loadScript(url,callback){  
        var script = document.createElement("script");  
        script.type="text/javascript";  
        if(script.readyState){  //IE
            script.onreadystatechange = function(){  
                if(script.readyState=="loaded"||script.readyState=="complete"){  
                    script.onreadystatechange=null;  
                    callback();  
                }  
            }  
        }else{  //Others: Firefox, Safari, Chrome, and Opera
            script.onload = function(){  
            callback();  
        }  
        }  
        script.src = url;  
        document.getElementsByTagName("head")[0].appendChild(script);  
    }